1. Field of the Invention
The present invention relates to a non-erasable type information recording medium (where information can be written only once) (for example, optical disks) and an information recording method thereof.
2. Description of the Prior Arts
Optical disks, where a large capacity of information can be recorded in high density, are widely used in an increasing number fields. There are two types of recording methods for optical disk: a non-erasable (write-once) type where information can be written only once; and an erasable type where information is erasable and rewritable. Of the above two types, the non-erasable type optical disks are often used as sales media for music, movies, etc., since the non-erasable type disks do not allow the re-writing of information and the costs thereof are generally restrained to be low.
Various kinds of information recording methods have been devised for non-erasable optical disks. In all of the methods, information is recorded by irradiating a narrowed laser beam on a recording layer of an optical disk to change the physical/chemical characteristics of the recording layer. The recording methods are roughly classified into two types where the following two modes are used, respectively: a heat mode where a change of the recording layer caused by a heating by laser beam is mainly used; and a photon mode where a change of the recording layer caused by the light (photon) of a laser beam is mainly used. In the heat mode recording method, which includes a punch recording method where tiny holes (pits) are made on the recording layer by a local heating by laser beam, inorganic materials such as compounds of Bi, Re and Te, chalcogenide, etc. whose melting and evaporating temperatures are low are mainly used. Moreover, in the heat mode method and the photon mode method, a refractive index/reflectance changing method is widely used where information is recorded by causing a change in the refractive index and reflectance of the recording layer. In this method, cyanic pigments are used as organic pigments, and chalcogenide, TeO.sub.2 -Te, etc., as inorganic materials.
As described above, various kinds of materials have been developed as materials for non-erasable optical disks. With respect to the inorganic materials, since the spin coating, which can be easily conducted on the organic materials, of the inorganic materials is difficult, an inefficient process such as evaporation, etc. is required in many cases to form a thin, uniform recording layer on the substrate of an optical disk. Moreover, with respect to the organic pigments such as cyanic pigments, etc., since the organic pigments tend to be decolored by light, it is required to use an optical triplet quencher and to make the organic pigments into metallic complexes to improve the stability, which complicates the manufacturing process.